Zeolites are microporous materials formed by TO4 tetrahedra (T=Si, Al, P, Ti, Ge, Sn, etc.) interconnected by oxygen atoms creating pores and cavities of uniform size and shape within the molecular range (3-15 Å).
These microporous crystalline materials may be used as catalysts in numerous chemical processes. The use of a zeolite with specific physico-chemical properties in a given chemical process is directly dependent on the nature of the reagents and products involved in the process (such as size, shape, hydrophobicity, etc.), as well as the reaction conditions. On the one hand, the nature of the reagents and products will affect the diffusion of these molecules in the pores and cavities of the zeolite and, consequently, the choice of a zeolite with a suitable pore topology for the products involved in the reaction is essential. On the other hand, the zeolite must be chemically and structurally stable under the required reaction conditions.
The formation of nitrogen oxides (NOx) during the combustion of fossil fuels has become a problem for society, since they are amongst the main air pollutants. The selective catalytic reduction (SCR) of NOx using ammonia as the reducing agent has become an efficient method for controlling said emissions (Brandenberger, et al. Catal. Rev. Sci. Eng., 2008, 50, 492).
Recently, it has been disclosed that silicoaluminates with the AEI structure and Cu atoms introduced therein present high catalytic activity and hydrothermal stability in the SCR reduction of NOx (Moliner et al. WO2013159825; Moliner et al. Chem. Commun., 2012, 2012, 48, 8264).
The AEI zeolite structure presents a tri-directional system of small pores (<4 Å) interconnected by large cavities, and also double six-membered rings (DA6) as secondary building units (Wagner, et al. J. Am. Chem. Soc., 2000, 122, 263).
The silicoaluminate form of the AEI zeolite structure can be synthesised using cyclic ammonium cations with alkyl substituents (Zones et al. U.S. Pat. No. 5,958,370; Cao et al. WO 2005/063624; Moliner et al. WO2013159825) or tetraalkylphosphonium cations (Sano et al. WO/2015/005369) as OSDAs.
In order to prepare the copper-containing silicoaluminate form of the AEI zeolite structure, the incorporation of the copper species is preferably performed by means of post-synthetic metal ion exchange processes on the previously synthesised and calcined AEI material (Moliner et al. WO2013159825; Sonoda, et al. J. Mater. Chem. A., 2015, 3, 857). When using this methodology, several steps are required to obtain the final material, including the hydrothermal synthesis of the silicoaluminate, calcination in order to eliminate the OSDA, transformation into the ammonium form, metal ion exchange and, finally, calcination, to obtain the material in the desired Cu-silicoaluminate form. All these steps contribute to increase the total cost of the material preparation process.
Therefore, the possibility of directly synthesising the material with the copper-containing silicoaluminate form of the AEI zeolite structure may considerably decrease the costs associated with the preparation thereof, since it would avoid most of the steps described above, making these directly prepared materials very attractive for industry.